The invention relates to a valve and to the use of the valve. Valves, for example for gas passages, are known. They often consist of an aluminum housing with a valve seat in which a rotatably arranged flap for closing the gas passage is arranged. To avoid leakage air, the valve is in this case provided with a piston seal encircling on the outside. On account of the requisite wear resistance, the valve seat is as a rule produced from steel in a ring shape. If the gases directed via the valve are at temperatures above 400° and if they also fluctuate greatly in their temperature, different expansions occur at the arranged flap and the valve seat, so that a certain annular gap is necessary between the flap and the valve seat during production. The leakage air which occurs as a result during operation can only be reduced in its quantity by very complicated production processes and by small tolerances being set.